The successful p-type doping of ZnSe using nitrogen radicals as a dopant during molecular beam epitaxy growth has rapidly lead to the demonstration of blue-green laser diodes. See for example R. M. Park et al, Appl. Phys. Lett. 57,2127, (1990); K. Ohkawa et al, Jpn. J. Appl. Phys. 30, L152 (1991); M. A. Haase et al, Appl. Phys. Lett. 59, 1273 (1991); and H. Leon et al, Appl. Phys. Lett 59, 3619 (1991).
Perhaps the most important obstacle preventing widespread use of these ZnSe-based blue-green lasers and maturing of this technology is the difficulty in fabricating low resistivity Ohmic contacts to p-type, normal thickness ZnSe-based cladding layers, eg, ZnSe, ZnS.sub.x Se.sub.1-x and Zn.sub.1-x Mg.sub.x S.sub.y Se.sub.1-y. Gold (Au) electrodes which are typically involved in the formation of electrical contacts, form a Schottky barrier on p-ZnSe in excess of 1.0 eV and no metal that has a work function that is high enough to lower that barrier has been found. Since ZnSe: N layers can be doped at most (so far) in the low 10.sup.18 cm.sup.-3 range, a realization of tunnelling metal-semiconductor junctions for electrical contact purposes is extremely difficult and has not been feasible. There is thus a need for an alternative solution to the problem. Reduction of the valence band affinity of the semiconductor near the surface by use of a graded gap semiconductor layer comes to mind as one method that may prove to be successful. Such a technique for producing Ohmic contact to n-GaAs has been reported. (See J. M. Woodall et al, J. Vac. Sci. Technol., 19, 626, (1981).:1.
Recently, several groups reported some progress in improving electrical contacts to p-ZnSe. For example,there have been developed: ZnSe:N contact layers by lowering the substrate temperature during molecular beam epitaxy growth to 150.degree. C., and using a cracked Se source (J. Qui et al, Proceedings of VII MBE Conference Schwabisch Gemund, Germany, Aug. 24-28, 1992); a ZnSe/ZnTe multilayer structure used for a contact layer to achieve a graded band gap effect from p-ZnSe to p-ZnTe (Y. Fan et al, Appl. Phys. Lett. 61, 3161, 1992); improved electrical contact to p-ZnSe by use of high affinity, semi-metallic HgSe, grown by MBE (Y. Lansari et al, Appl. Phys. Lett. 61,2554, 1992. Although these improved contacts permitted performing Hall measurements on p-ZnSe, to date they have not reduced the operating voltage of blue-green lasers to any significant extent. While the exact reason is not known, perhaps due to the large lattice mismatch with GaAs substrate, the dislocated ZnTe, ZnTe/ZnSe, and HgSe contacting layers are not compatible with laser structures operating at high current conditions. In addition, ZnTe-ZnSe layers have a higher refractive index than the active region formed by ZnSe or ZnCd.sub.x Se.sub.1-x layers, which has a detrimental effect on the optical confinement in the active region of the laser. To compensate for this effect, an extra-thick p-cladding layer would be required which further complicates matters since it is likely to increase the voltage threshold for laser emission.